IG - Index Generation

Enhanced Definition

IG (Index Generation) refers to the process of creating or rebuilding an **Alternate Index (AIX)** for a **VSAM Key -Sequenced Data Set (KSDS)**. This process is typically performed using the `BLDINDEX` command within the **IDCAMS** utility, which reads the base KSDS and populates the AIX with keys and pointers to the corresponding base records. Its primary purpose is to provide alternative access paths to data stored in a KSDS.

Key Characteristics

- Utility: Primarily executed using the BLDINDEX command of the IDCAMS (Access Method Services) utility.
- Input/Output: It reads the base VSAM KSDS and writes the alternate key and its corresponding base key (or Relative Byte Address - RBA) to the Alternate Index (AIX) data set.
- Purpose: Enables applications to access records in a KSDS by a key other than its primary key, providing alternative logical views of the data.
- Types of AIX: Can create both unique and non-unique alternate indexes, depending on whether the alternate key values in the base KSDS are unique.
- Performance Impact: While providing flexible access, maintaining AIXs adds overhead to update operations on the base KSDS, as both the KSDS and all associated AIXs must be updated.
- Integrity: Ensures the alternate index is synchronized with the base KSDS at the time of generation or rebuild, maintaining data consistency.

Use Cases

- Creating a New Alternate Index: After defining a new AIX for an existing KSDS, BLDINDEX is used to populate it for the first time with data from the base KSDS.
- Rebuilding a Corrupted AIX: If an AIX becomes logically inconsistent with its base KSDS due to system failures, software errors, or data corruption, BLDINDEX can be used to rebuild it from scratch, ensuring data integrity.
- Performance Optimization: For applications frequently querying a KSDS on a field other than its primary key, an AIX can significantly improve retrieval performance by avoiding full data set scans.
- Adding New Access Paths: When business requirements evolve, and new ways to access existing data are needed, an AIX can be defined and built without altering the base KSDS structure or existing applications.

Related Concepts

IG is intrinsically linked to VSAM KSDS as it operates on these data sets to create Alternate Indexes (AIXs). An AIX itself is a type of VSAM KSDS, but its records contain alternate keys and pointers (either base KSDS primary keys or RBAs) to the records in the base KSDS. The IDCAMS utility is the primary tool used for IG, and the resulting AIX is often accessed by application programs (e.g., COBOL programs) via a Path, which logically connects the AIX to the base KSDS.

Best Practices:

Careful Design: Define alternate keys judiciously, considering their uniqueness, length, and offset within the record, as these factors significantly impact storage, I/O, and overall performance.
Scheduled Rebuilds: For heavily updated KSDSs, periodically rebuilding AIXs can help reclaim space and improve access performance, especially if BLDINDEX is used with the REUSE option.
Monitor Performance: Be aware of the overhead introduced by AIXs during update operations on the base KSDS; too many AIXs or poorly designed ones can degrade overall system performance.
JCL for BLDINDEX: Ensure the JCL for BLDINDEX includes appropriate DD statements for the